1. Field of the Invention
The present invention relates to an igniter semiconductor device connected to a primary side of an ignition coil and capable of turning on and off current flowing in the primary side, an igniter system having the igniter semiconductor device, and an ignition coil unit having the igniter semiconductor device.
2. Description of the Related Art
As an ignition device of an internal-combustion engine, an igniter system that turns on and off current flowing in a primary side of an ignition coil using a semiconductor device and allows an ignition plug to spark using high voltage generated on a secondary side of the ignition coil by electromagnetic induction is used. The igniter system comes in two systems. One is a distributer system in which an ignition coil and an igniter semiconductor device are disposed in an electronic control unit (ECU) and power is distributed to ignition plugs of respective cylinders by a mechanical mechanism. The other is an individual ignition system in which an ignition coil and an igniter semiconductor device are disposed in each ignition plug and the ignition time is adjusted for each cylinder. When the individual ignition system is employed, the igniter semiconductor device is required to have better heat resistance and reliability than the distributer system since the igniter semiconductor device is exposed to a severe environment such as temperature stress, electrical stress, and mechanical stress caused by vibration.
Conventionally, a hybrid igniter semiconductor device in which a switching element chip and a protection circuit chip are combined has been used in order to protect the device from overheat, overcurrent, and abnormal current (surge current). However, recently, a small igniter semiconductor device in which the switching element and the protection circuit are disposed on the same chip has been used.
For example, Japanese Patent Application Laid-open No. 2012-207669 discloses an igniter semiconductor device (one-chip igniter) in which an overheat protection circuit and an overcurrent protection circuit are disposed on the same chip.
Hereinafter, a conventional one-chip igniter will be described with reference to the drawings.
FIG. 7 illustrates a circuit configuration of an igniter semiconductor device 103. The igniter semiconductor device 103 includes an input terminal 2, an output terminal 3, and a ground terminal 4, through which the igniter semiconductor device 103 can be connected to an external device. The igniter semiconductor device 103 includes a switching element 5, an overheat detection circuit 8, an overcurrent detection circuit 9, pull-down NMOSs 10 and 11, a sense resistor 12, a collector-gate zener diode (CGZD) 13, a surge protection zener diode 14, a waveform shaping circuit 15, a protection diode D1, and a pull-down resistor R1, which are mounted on one chip. In this example, the switching element 5 is an insulated gate bipolar transistor (IGBT).
The input terminal 2 is connected to a gate electrode of the switching element 5. The output terminal 3 is connected to a collector electrode of the switching element 5. Moreover, the ground terminal 4 is connected to an emitter electrode of the switching element 5. When the voltage input to the input terminal 2 is higher than a threshold voltage of the switching element 5, the switching element 5 is turned on and current flows between the output terminal 3 and the ground terminal 4. When the voltage is lower than the threshold voltage, the switching element 5 is turned off and no current flows between the output terminal 3 and the ground terminal 4.
The overheat detection circuit 8 and the overcurrent detection circuit 9 are connected to a signal line that connects the input terminal 2 and the gate electrode of the switching element 5. An ON signal is a signal for turning on the switching element 5 and is also used as a power supply that supplies power to the overheat detection circuit 8 and the overcurrent detection circuit 9.
FIG. 8 illustrates a circuit configuration of an igniter system 1003 which uses the igniter semiconductor device 103. The igniter system 1003 includes an electronic control unit 203, an ignition coil unit 303, and an ignition plug 40.
The electronic control unit 203 includes a regulated power supply circuit 50 and a signal control circuit 20. The regulated power supply circuit 50 can create a power supply voltage VCC for control circuits from a battery voltage VBAT of battery 60 and supply power to the signal control circuit 20. The signal control circuit 20 includes an inverter circuit made up of a PMOS 21, a NMOS 22, and a series resistor 23. The signal control circuit 20 can control the igniter semiconductor device 103 connected via a wire L2 using the power supply voltage VCC as a power supply.
The ignition coil unit 303 includes the igniter semiconductor device 103, an ignition coil 31, and a diode 32. A positive terminal on a primary side of the ignition coil 31 is connected to the battery 60. A negative terminal on the primary side of the ignition coil 31 is connected to the output terminal 3 of the igniter semiconductor device 103 through a wire L3. A positive terminal on a secondary side of the ignition coil 31 is connected to the battery 60. A negative terminal on the secondary side of the ignition coil 31 is connected to the ignition plug 40 through the diode 32.
When an ON signal is transmitted from the signal control circuit 20 to the input terminal 2 through the wire L2, the igniter semiconductor device 103 is turned on and current flows to the primary side of the ignition coil 31. Subsequently, when an OFF signal is transmitted from the signal control circuit 20 to the input terminal 2 through the wire L2, the igniter semiconductor device 103 is turned off, the current on the primary side of the ignition coil 31 is blocked, and high voltage is generated on the secondary side of the ignition coil 31 due to electromagnetic induction. As a result, the diode 32 breaks down, the high voltage is applied to the ignition plug 40, and the ignition plug 40 sparks.
As illustrated in FIG. 7, in the conventional igniter semiconductor device 103, a driving circuit is not provided between the input terminal 2 and the gate electrode of the switching element 5. On the other hand, the pull-down resistor R1, the overheat detection circuit 8, and the overcurrent detection circuit 9 are connected to the input terminal 2, and an ON signal current flows continuously in the input terminal 2 during the ON period of the switching element 5. Since the ON signal current flows continuously, the gate voltage of the switching element 5 falls in the PMOS 21, the series resistor 23, and the wire L2 disposed in the path of the ON signal current and becomes lower than the power supply voltage VCC. When the power supply voltage VCC decreases greatly due to the influence of an external environment, the gate voltage becomes lower than the threshold voltage. Thus, the igniter semiconductor device 103 may not be turned on and an ignition failure may occur.